High voltage arc plasma generator



SEARCH RM July 13, 1965 R.J.BA1RD 3,194,941

HIGH VOLTAGE ARC PLASMA GENERATOR Filed sept. 1s, 1962 \O` N 'mw U1 Q iIl TQ I N l "1 Q"`` Q z "`-`N l I N *N 1- N r """OQ OO J\` Q I \f 1INVENTOR ROBERT- J. RnB-MRD BY 'l United States Patent 3,194,941 HIGHVOLTAGE ARC PLASMA GENERATOR Robert J. Baird, Indianapolis, Ind.,assignor to Union Carbide Corporation, a corporation of New York FiledSept. 13, 1962, Ser. No. 223,484 4 Claims. (Cl. 219-121) This inventionrelates to an improved method and apparatus for ,obtaining a highvoltage arc plasma and more particularly to such a high Voltage arcplasma established in a system wherein material to be treated by sucharc plasma is part of the electrcial circuit.

Electric arcs have been used for many years for cutting, for plating andwelding processes and of recent years for metal melting furnaces, metalscarling, and metal cutting. In applications such as these, it isoftentimes preferable to operate the arc in a transferred mode; that is,having the work in the arc circuit. This mode of operation has theadvantage of more fully utilizing the heat energy -of the arc in theworkpiece rather than having a portion of it dissipated to a separateelectrode.

Further, in such applications, it is of prime importance to get themaximum amount of the power being supplied to the arc-generator deviceto be transferred to the gas and the work. It has been found that ifhigher power to the arc device is achieved solely through currentincreases, such additional power is used up primarily in heating theelectrode and the cooling fluid streams. On the other hand, higher powerobtained by voltage increases is substantially transmitted as higherheat to the arc gas and the work.

I-t is the main object of the invention to provide a method 4andapparatus for producing a high voltage arc plasma for use in electricarc working of materials.

It is a further object to provide a novel high voltage device capable ofoperating in a transferred mode.

It is still another object to provide a novel high voltage device whichis preferably operated with alternating current.

These and other objects will either be pointed out or become apparentfrom the accompanying description and drawings wherein the sole ligureis a cross-section View of the device of the invention.

In -a general way, the objects of the invention are accomplished byproviding an -apparatus having a cup shaped electrode, a gas directingnozzle having an L/ LD. of at least about 1.2 spaced from the cup shapedelectrode, and a chamber surrounding the space between the cup electrodeand the nozzle and having means for introducing `an arc gas into thechamber to produce a vortical ilow in the chamber and in the gasdirecting nozzle. For purposes of this disclosure, the length (L) of thenozzle is measured from the nozzle exit to end of the chamber nearestthe exit. The inside diameter (LD.) is the minimum inside diameter of`the nozzle.

Further, the invention provides a novel process for generating .a highvoltage, high temperature arc plasma for working materials. In thisprocess, a cup shaped electrode and the material to be worked areconnected in circuit relation. A high voltage arc is established betweenthe cup-electrode and the material to be worked. A vortical ilow of arcgas is provided in the region of the arc to form .an arc plasma. The arcplasma is then passed through a gas directing nozzle having an L/I.D. ofat least 'about 1.2. The vortical flow of arc gas is maintained insidethe gas directing nozzle to fcollimate `and direct the arc plasma. Thenthe so-collimated and directed arc palsma is applied to the material tobe treated.

The method and apparatus of the invention fulfills a long felt need for-a high power high voltage electric arc plasma generator for use in atransferred mode and having minimum electrode erosion.

Referring now to the drawings, the torch T primarily consi-sts of a cupshaped electrode 14 that is in axial alignment with a gas directingnozzle 16 and which is separated therefrom by an arc chamber 10. Chamber10 is electrically insulated from the electrode 14 by insulator 11. Arcgas is introduced to torch T through inlet 13. From inlet 13 the arc gaspasses through passage 18 down to a plurality of tangential apertures12, through such apertures into the chamber 10. The torch T is cooled bypassing -a cooling fluid from coolant inlet 20 through passage 22 into asecond passage 24 formed between the nozzle 16 and member 26. Frompassage 24 the coolant passes from the torch through chamber 28 andpassage 30- and outlet 32.

The electrode 14 is also cooled by introducing a coolant through inlet34 down passage 36 -through passage 38 down passage 39 and back uppassage 41 through cross-passage 43 and out passage 40. The outertubular member 42 which deiines the passage 36 also carries means forconnecting a power supply to the torch T. Therefore such member 42 isinsulated from the remainder of torch T by insulator 44.

In actual operation, a quantity of arc gas is introduced into chamber 10through apertures 12 so as lto impart a swirling or vortex motion to thegas. The general configuration of the torch permits part of the g-as toliow into and out of the rear electrode 14 and -then all of the gasthrough the nozzle 16 so long as the ga-s is introduced at a suflicientvelocity. That is, if the inlet velocity is greater than 0.25 Mach, thenthere will be a sufficient pressure drop within the chamber 10 -betweenits outer wall 60 and the area near the torch axis to force asubstantial portion of the gas into ele-ctrode 14. This results in -alonger arc which increases the arc voltage. This pressure drop also aidsin maintaining a vortex flow of gas.

An appropriate power supply (not shown) is connected to the cupelectrode 14 Iand to a workpiece. The workpiece will, of course, varyaccording to the application for which the torch is being used. Forexample, it may be .a metal plate that is being scarfed, or it may bethe charge of a metal melting furnace. Whatever it may be, thecombination of the high voltage arc operating in a transferred mannerrenders this torch extremely useful for such applications.

The arc is initiated by .any .suitable means, such as for example ahigh-frequency start, capacitor discharge or by inserting a conductingrod into the torch through the nozzle. The quantity of gas is thenincreased as desired. The arc that is finally established will, becauseof the flow of gas in the torch, ex-tend from Van area appreciably alongthe length of the cup electrode, through nozzle 16, to an electrodeworkpiece.

In order to successfully operate the torch of the invention, theconfiguration of Ithe nozzle 16, which constricts and directs the arcand aids lin increasing the arc voltage, is critical. It has been'foundthat if the .ratio of the length (L) of the nozzle -to the insidediameter (LD.) becomes too small, -there will be an insuicient radialpressure gradient within the nozzle to center lthe arc. Consequently,the -arc will strike to the nozzle and then to the work; this phenomenonis known as double arcing. It has been found that if the L/I.D. is less.than 1.2, the low pressure region toward the center of the nozzle willbe decreased, thus causing double arcing and severe nozzle erosion. Ont-he other hand, an L/LD. much greater than 1.2 makes it more difficultto transfer the `arc and reduces the heat efficiency of the arceiliuent. It is desirable, therefore, that the L/I.D. of

the nozzle be between about 1.2 and 3.0 and preferably about 2.

While direct current with straight or reverse polarity connection may beused with this device, it is preferred not only from the standpoint ofachieving higher power levels at lower costs, but also from thestandpoint of longer life for the cup electrode to operate the torchwith single phase alternating current. For example, when operated withdirect current, the arc tends to localize over a particular area of thecup-shaped electrode, whereas, with alternating current the arc will runalong substantially the entire length of the electrode. More specically,the arc termination will alternate between an area somewhere near theclosed end of the cup-shaped electrode and its mouth on every half cycleof the alternating current. It should be emphasized that suchalternating or traveling of the arc has been made possible to a greatextent by the particular configuration of the torch which, because ofthe intermediate chamber, forces a portion of the gas entering thedevice to travel into and out of the cup-shaped electrode. Thissubstantially reduces erosion of the cup electrode. Such an electrodenot only reduces material usage, but also prevents contamination of thework for which the torch is being used.

As an alternate means of providing for longer electrode life through theprevention of electrode erosion, a watercooled copper field coil couldbe placed around the cup electrode so as to produce a magnetic field,thus causing the arc to rotate. Such coil will not only cause the arc torotate, but it will also spread out the arc on the electrode so as topermit greater overall currents with workable current densities.

Various electrode materials have been found to be useful with varioustypes of gases. Thus, copper, silver, aluminum, zirconium, andmolybdenum are useful materials when the device is operated withreactive gases such as air, oxygen, carbon dioxide, and carbon monoxide.Such material is useful to minimize electrode damage in the presence ofoxidizing atmospheres. The material is also useful for mixtures of aninert gas with air, oxygen, or carbon monoxide. When inert gases such ashydrogen, argon, helium, and nitrogen are being used, tungsten, tungstencontaining emissive material such as thoria, and carbon is the preferredelectrode material.

Suitable materials for the insulator have been found to bephenol-aldehyde condensation resins and nylon. However, materialsexhibiting like properties could also be used.

The following examples indicate the utility of this transferred, highvoltage torch. of the general type depicted in the drawing was used.

EXAMPLE 1 Oxidation scarfing In this example, the cup electrode had alength of 91/2 in. and an I.D. of 1% in. The nozzle had a length of 2.75in. and an I.D. of 0.951 in. The workpiece, that is the other electrode,consisted of a type 304 stainless steel bar approximately 4 in. wide, 2ft. long, and 3%: in. thick. Oxygen was supplied to the device at therate of 1000 c.f.h. so as to cause oxidation of the metal, thus,permitting the defects of the metal to be blown away. A tield coilhaving a magnetomotive force of 16.5 kilo-ampere turns was placed aroundthe rear electrode, the direction of the field being toward the bottomof the cup electrode. With the workpiece acting as the cathode, 665amperes (D.C.) were supplied to the device. The arc voltage was 285volts. The total power to the torch was 190 k.w. of which approximately152 kw. went to the gas and to the workpiece yielding an efficiency ofapproximately 80 percent.

The torch was set at an angle of 55 degs., the nozzle being directedopposite the direction of travel of the workpiece. The stando@ distanceof the torch from the In these examples, apparatus workpiece wasapproximately ll/z in. The workpiece had a travel speed of approximately250 in./min.

Under these conditions, a scarf having a width of approximately 0.94 in.and a depth of 0.09 in. was made. The scarfed area had a surface qualityjudged rollable based on past experience with this problem.

EXAMPLE 2 Fusion scarfing The dimensions of the torch used in thisexample were the same as those of Example 1. Likewise, the torch angleand the standoff distance were the same. The travel speed of theworkpiece was 50 i.p.m. The workpiece electrode was again a 304stainless steel bar of approximately the same dimensions. Again a fieldcoil was used, the magnetomotive force this time being 14.3 kiloampereturns, the direction of the r'ield again being toward the bottom of therear electrode. However, in this example, argon was supplied to thetorch at the rate of 2000 c.f.h. so as to melt the metal to remove thesurface defects. In this case the metal is not blown away, rather itsolidities.

With the workpiece acting as the anode, 550 amperes (D.C.) were suppliedto the device. The arc voltage was 210 volts. The total power to thetorch was 115 kw. of which approximately 92 kw. went to the gas and tothe workpiece yielding a torch efficiency of approximately percent.

Under these conditions, a scarf having a melt width of approximately0.63 in. and a depth of 0.04 in. was made. The surface quality was good.

EXAMPLE 3 Metal Cutting In this example, the cup electrode had a lengthof 61A in. and an I.D. of 1/2 in. The nozzle had a constriction having alength of 1/2 in. and an I.D. of M1, in. Oxygen was supplied to thedevice at the rate of 750 c.f.h. The electrode workpiece was an 8 in.stainless steel plate. With the workpiece acting as the cathode, 300amperes were supplied to the device. The arc voltage was 650 volts. Thetotal power to the torch was 195 kw. of which approximately 154 kw. wentto the gas and to the workpiece yielding a torch eiciency ofapproximately 79 percent. The torch was directed at the workpiece at anangle of degs. from a standoff distance of 1/2 in.

Under these conditions a cut 5 in. in length was made through the plateat the rate of 3 i.p.m. The qualtity of the cut was judged to be good.

EXAMPLE 4 Metal melting In this example, the cup electrode had a lengthof 10 in. and an LD. of 1% in. The nozzle had a length of 21/2 in. andan I.D. of 1% in. The workpiece electrode consisted of 1500 lbs. ofcarbon steel scrap. Air was supplied to the torch at a rate of 600c.f.h. Using single phase alternating current, 1100 amperes weresupplied to the torch. The arc voltage was 400 volts. The total power tothe device was 440 kw. of which 362 kw. went to the gas and to theworkpiece during the initial run yielding an etliciency of approximately82 percent. As the environment surrounding the torch became hot (1600degs. C.), the torch etliciency dropped to 40 percent due to inadequateexternal thermal insulation of the torch.

Under these conditions the scrap was converted to a molten bath in 21/2to 3 hours.

The ability of the torch to operate on alternating current in air torender the cup electrode substantially nonconsumable can be shown.

EXAMPLE 5 A Ifernatng current operation in air In this example the cupelectrode had a length of 9% in. and an I.D. of 1% in. The nozzle had alength of 2% in. and an I.D. of 1% in. The electrode workpiece was aplate of carbon steel. Air was supplied to the torch at the rate of 500c.f.h. Using single phase, alternating current 1200 amperes weresupplied to the device. The arc voltage was 320 volts. The chamberpressure was approximately 1 atmosphere. The total power to the torchwas 375 kw. of which 300 kw. went to the gas and to the workpieceyielding an eiciency of 80 percent.

Under these conditions, the torch operated for a period of about 10minutes without any appreciable signs of electrode erosion.

What is claimed is:

1. Apparatus for producing a high-Voltage high-temperature arc plasmabetween such apparatus and a metal connected in arc circuit relationtherewith which comprises a cup-shaped electrode, a gas-directing nozzlehaving a L/I.D. of at least about 1.2, said nozzle being in axialalignment with but spaced from said cup-shaped electrode, a chambersurrounding the space between said cup-shaped electrode and nozzle andhaving means for introducing an arc gas into such chamber to produce avortical ow in such chamber and said gas-directing nozzle.

2. Apparatus according to claim 1 wherein said gasdirecting nozzle has aL/I.D. of between about 1.2 and 3.0.

3. Apparatus according to claim 1 wherein said gasdirecting nozzle has aL/ I D. of about 2.

4. Apparatus for producing a high-voltage high-temperature arc plasmabetween such apparatus and a metal connected in arc circuit relationtherewith which comprises a cup-shaped electrode, a gas-directing nozzlehaving a L/I.D. of at least about 1.2, said nozzle being in axialalignment with but spaced from said cup-shaped electrode, a chambersurrounding the space between said cup-shaped electrode and nozzle andbeing provided with means positioned between the cup electrode and thenozzle for introducing arc gas into such chamber directly into the spacebetween said cup-shaped electrode and nozzle to produce a vortical ow insuch chamber and said gas directing nozzle.

References Cited bythe Examiner UNITED STATES PATENTS 3,118,046 1/ 64Harrington 219-75 3,131,288 4/64 Browning.

RICHARD M. WOOD, Primary Examiner.

JOSEPH V. TRUHE, Examiner.

1. APPARATUS FOR PRODUCING A HIGH-VOLTAGE HIGH-TEMPERATURE ARC PLASMABETWEEN SUCH APPARATUS AND A METAL CONNECTED IN ARC CIRCUIT RELATIONTHEREWITH WHICH COMPRISES A CUP-SHAPED ELECTRODE, A GAS-DIRECTING NOZZLEHAVING A L/I.D. OF AT LEAST ABOUT 1.2, SAID NOZZLE BEING IN AXIALALIGNMENT WITH BUT SPACED FROM SAID CUP-SHAPED ELECTRODE, A CHAMBERSURROUNDING THE SPACE BETWEEN SAID CUP-SHAPED ELECTRODE AND NOZZLE ANDHAVING MEANS FOR INTRODUCING AN ARC GAS INTO SUCH CHAMBER TO PRODUCE AVORTICAL FLOW IN SUCH CHAMBER AND SAID GAS-DIRECTING NOZZLE.